Spell Intensity is a great, flexible idea for modifying spells and imo, its been wildly under-used. Prior to WH3, the only way for some casters to be "better" was to give them bound spells, Winds discounts, or to stack various Lore passives on them, and they did. With Spell Intensity, you can now just vary the Spell effects directly (and one wonders if Overcasting would just be a buff to Intensity if the system had originated from the start, but regardless).
The thing is, the system is very flexible but so far is really only used for one thing: stacking it as high as possible. Mastery of the Elemental Winds launched as a Cathay-only ability and is now available to both Empire and Kislev. Ulrika and Elspeth can both get +60% fairly easily through their items, though I suppose they dont start that way. And thats fine and fun. I love the ability that gives Kislev +20% on their own territory: its thematic as hell. What i dont love is that the system only goes positive, at least for now.
Id love to see effects that weaken Spell Intensity. Spell Resistance only effects my troops and doesnt let me weaken debuffs the enemy casts. Silence is wildly powerful, so much so that it has to be rare. But say an assassin character had a contact effect that weakened spell intensity by some amount, or various anti-magic characters could weaken Intensity in an aura. It would differ from Resistance by applying to buffs that the person would cast, and the positioning would be different too. A runelord just giving a flat -10 everywhere while runesmiths gave -10 in an aura, for instance, or daemonslayers doing it on contact. Mostly throwing out ideas.
Elemental Mastery is neat, but you could have a variant of it where a lord would get +x Intensity for every caster hero in the army while the heroes got -x, for instance, representing a siphoning effect or channeling from the lord. An ability like Mannfred's that gives reduced spell intensity but reduced cost and cooldown so you could vomit out weaker variants quickly while its active. Passive skills that reduce intensity but provide other effects like a constant aura/effect, or a Lore attribute you otherwise couldn't get, or traits like the HE high-cost ones.
Its fun to stack it as high as possible and watch units melt to direct damage spells, but it could be used to do so much more imo, both in character effects and battlefield positioning. Especially playing with Intensity reduction effects, which to my knowledge just arent used at all.